Abstract: The interrelationships between four quantitatively definable characteristics of an activated sludge biomass and three selectable engineering control variables were examined computationally for their effect on effluent quality, reactor biomass, and excess sludge production. It was concluded that the model offers both the designer and the operator a means of predicting and controlling performance of the process. Laboratory pilot plant studies in support of the model has shown its stability in maintaining a steady state and its ability in predicting performance of the process. The characteristic properties of the biomass are true cell yield, maintenance coefficient, and the Monod constants, maximum specific growth rate, and saturation constant. The controllable engineering parameters are cell recycle concentration, hydraulic recycle ratio, and mean hydraulic detention time or dilution rate.